My attempts to make a rapid prototyping machine that I will use to make parts for a machine that will be able to make parts for a copy of itself.

Tuesday, 28 September 2010

Rebore

My Mendel has been very reliable and consistent running virtually 24/7, but about a week ago, after putting on a new reel of plastic things started to go wrong. The initial symptoms were that small parts built fine, in fact I printed a mini Mendel or Huxley that came out well: -

It took just two full Mendel beds, plus a few parts on HydraRaptor. I did the gears on Hydra for accuracy and the Bowden clamps at 100% fill because they look weak to me for the job they are intended to do. The plastic weighs 335g (including a Wade's extruder), slightly more than 1/3 of a Mendel by weight but the printtime is about 1/2, because small parts need finer filament. I printed most of these at 0.5mm whereas I do a lot of Mendel at 0.6mm.

But getting back to the problem, the quality of large parts had started to fall off a bit. They were coming out with blobs on the outside formed by the nozzle oozing as it moves from one object to another. These were not well bonded, so they could be simply scraped off with a fingernail, but something I had tuned out ages ago. Another change was that it was not doing 45 degree overhangs well, so it left filament hanging down in the tops of tear shaped holes. Again, not a big problem as they just get drilled out anyway.

I started to suspect the temperature was too high so I pushed the thermistor well into the heater block. Then the filament started jamming after the first layer (which I do very slowly). After a few attempts the extruder drive gear broke where the captive nut for the grub screw is. This seemed more like the temperature was too low, so I suspected the thermistor was no longer reliable. I decided to rebuild the heater assembly as my last one was put together in a hurry from parts left over from an experiment. It had been in the wars as well, being entombed in ABS and hacked out again, not to mention running almost continuously for about 2500 hours. Originally the thermistor was glued in with RTV silicone, but that was long gone and it relied on the wires holding it in place.

Since my original heater hack using a vitreous enamel resistor I had moved on to a smaller resistor on Hydra and found that worked better. The surface area of the block is a lot less and that is where most of the heat is lost from, so the amount of power required goes down. It also warms up faster of course, both due to less heat being lost and also less thermal mass. The resistor I have settled on is a Vishay / Sfernice RWM04106R80JR15E1

The thermistor is drilled as close as I dare to the thread for the nozzle and then counter-bored so that the entrance is wide enough for the PTFE sleeving. The wires have PTFE insulation to withstand the temperature and the resistor is soldered with 300°C HMP solder. I think I could also get away with ordinary unleaded solder as well because of the length of the resistor leads, but I didn't want to chance it.

After a tip from Giles I used Rothenberger high temperature glass rope adhesive to glue the resistor and the thermistor. It sets in only half an hour, which is a big advantage over other things I have tried. I also used it to stick ceramic tape on the outside of the block to insulate it.

When I first heated it up the adhesive bubbled causing a downward slope in the temperature graph. I thought at first the thermistor had been dislodged by the blistering, but I think it was just temporarily cooled by the out-gassing. I should have heated it much more slowly the first time I think.

The new heater works much better than the old one. The warm up time to 255°C is about 280 seconds, whereas the old one took about 400 seconds (the bed takes about 350 seconds to get to 140°C). It also runs at about 70% to maintain 240°C while extruding, whereas the old one needed about 90%. The bang-bang control cycles much faster and only deviates by one degree. That is because of the close proximity of the thermistor to the heater. Because it is mounted between the heater and the barrel I can be sure the swing at the barrel is even less. I calibrate against a thermocouple inside the barrel, so any temperature difference across the block is calibrated out. It should be negligible though because the thermistor is also very close to the barrel and aluminium is a very good conductor. The extra power needed to heat the ABS when extruding 0.6mm filament at 32mm/s is about 10%, i.e. ~2W.

The new improved heater didn't solve any of my problems though. While reassembling the extruder I tried pushing filament through by hand. It was much harder than I remembered it was when I first built the extruder. At this point I was beginning to suspect the plastic was different in some way although it looked identical and was part of the same purchase.

I noted that the filament was coming out very curly. That was something I had noticed happening on both my machines when I do a test extrusion, but I had ignored it. I measured the diameter though and found whereas it normally swells to 0.7mm this was coming out oval and about 0.5mm by 0.6mm. It all fell into place then. I have read that the difference between straight hair and curly hair is whether it is round or oval. The only way the filament could be oval is if the nozzle aperture is no longer round. I put a 0.5mm drill bit through it and it started to extrude round, straight, 0.7mm filament again. The hole must have been partially occluded by the burnt plastic that tends to glaze the end of the nozzle. That caused the plastic to come out thinner and faster. It was fine when making objects with 0.5mm filament because it was still being stretched but when building with 0.6mm filament it was being compressed, so would hang loose if given the chance. The smaller hole increased the barrel pressure, which is why it oozed. The plastic would be compressed more, so require more backing up to release the pressure and stop the flow. Also the extra pressure was too much for the pinch wheel when extruding at the top flow rate I use, which is 0.6mm at 32mm/s. I think the M8 hobbed bolt is below the ideal diameter for softer plastic like ABS.

I also re-bored HydraRaptor (with a 0.4mm drill) and that stopped the filament being curly as well. It seems nozzles need occasionally re-boring. I had assumed that the hot flow of high pressure plastic would have kept the hole clean, but not so.

So a simple fault had my machine out of action for days because I didn't recognise what the symptoms meant collectively.

35 comments:

I have gone off using glue now, the high temp glass rope glue I was using seemed to be strange stuff, and when it got hot it would crumble and the thermistor would fall out. Maybe it has to be "cured" or something. Two alternative things I tried: 1) a split heater block that can hold the resistor using a mechanical clamp and 2) a thermistor clamp made from ptfe

Hi Giles, Bad news about the glue. What is it about these high temperature glues? I found out why JB-Weld is no good: The max temperature rating is only for 10 minutes!

The resistors are a bit lumpy and fragile so I think you need something compliant to fill the gap, even with a clamp. I think I would warp it in several layers of AL foil to cushion it. That would allow you to have the clamp fully closed.

I have used a PTFE clamp like that for the thermocouple on my bed. I may switch to a thermocouple for the extruder at some point to avoid the need to calibrate it. The problem I see is the wires aren't flexible enough for moving head.

I built one of these with two resistors using parts from the makergear kit based on your designs. It heats to the set temperature (for PLA) in about 2 minutes. It came out really ugly because I didn't have a drill press, but it works great. http://www.flickr.com/photos/7820200@N05/5011601625/

I was also having problems with extrusion (couldn't push it through by hand, it didn't even get to the nozzle), which I solved by drilling my barrel out to 1/8". I've learned my lesson about trying to clean out the barrel by heating it with a butane torch until all of the PLA flows out. By the time all of it is hot enough to flow freely, metal is hot enough to burn it and gunk up the walls.

Yes I thought about two but the bigger it is the more power it needs, so it is a law of diminishing returns. At one extreme is HydraRaptor with only one of these resistors but with an integrated nozzle, no heater barrel and a PTFE cover. That warms up to 250C in two minutes and runs at 45% power driven by a small MOSFET with no heatsink. At the other extreme is the Makerbot MK5 with two massive AL clad resistors that uses an enormous amount of power fed by a relay.

Yeah, thing is though, the kit I used parts from was designed to be used with one resistor. I just drilled out a second resistor hole, so it's not any bigger than the one I built before from the same kit with one resistor. It uses twice as much current, but it doesn't have any more thermal inertia to overcome. I haven't done the tests, but I assume it runs at about half the duty cycle of the old one to maintain temperature. If everything is custom-machined, of course further optimization will be possible.

OTOH, the less aluminum you use, the more having two resistors on opposite sides of the barrel will help to ensure that it is heated evenly. You also probably don't want it too small. Thermal inertia makes in take longer to heat, but it also stabilizes the temperature when it's hot.

At the end of the day, my old one wasn't working great and eventually stopped getting the job done, and the new one works really well. For the tools and materials I had at my disposal, it turned out pretty well.

What thermistor would you order from Farnell's to go together with the resistor? Also do you have any suggestions for RS - resistor & thermcouple. The reason I ask is that I may find it easier to make the minimum order for RS rather than Farnell as the Mendal BOM has lots of RS parts to order.

I use an Epcos 10K thermistor from RS which is not suitable for the RepRap 5V electronics. You can get the RepRap 100K thermistor from either RS or Farnell, but it seems to be US stock only at Farnell. That seems to be increasingly the case.

You can get exactly the same resistor from RS http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=4851672

@Keith The £20 minimum order thing at Farnell is a pain. Fortunately I live near by to their trade counter so I just pop in and pay cash. I was going to pick up some resistors and thermistors from Farnell this weekend, if you are interested I can pick up a few spare and sell them on through ebay.

Giles: Thanks I am just after building an extruder for a CNC machine at this stage. I also have an RS Trade counter not too far away. Will bare in mind your comments when I get to the stage of getting bits for a Mendal.

This has been printing continuously since the beginning of May, so once a month is probably going to be excessive. I don't know if the inside of the barrel needs it or not. I have no easy way to empty it and see inside. I assume that since the air is excluded and the plastic isn't stationary for long, that it doesn't get burnt inside, just at the nozzle entrance.

Also I am not sure if solvent will remove it after it has burnt. It is a lot easier to just clear the nozzle with a drill bit. I will report if I ever need to clean out the barrel in the future.

I think coating the nozzle with PTFE would avoid this problem, but probably not something that can be done at home.

Giles Belts costs can be cut extremly if you buy from a beltfirm and buy 1 "thick" belt and later cut them using the splitter rig to multiple belts (and when i mean thick i dont mean for 1 reprap) difference in cost for a 10mm belt instead of a 15mm is about less then 1£ of course the prices vary depending on retailer mine beeing a beltfirm in stockholm swedem.

Well the glass rope adhesive is holding it at the moment. Perhaps if it is cured slowly so that it doesn't blister it will work. I don't think it is very strong, but once my extruder is assembled there is very little mechanical stress on it.

It is still working well, but I won't know how much strength it has until I disturb my extruder.

I think perhaps the issue is that is says it sets enough to close the oven door in 30 minutes, which it does, but I don't think it gets fully cured until you put it through a heat cycle. That needs to be done slowly to avoid it blistering.

While Farnell mention a minimum order on their website, I've never had any problems with small orders before (most of my orders have been under £10, some under £2, all with free shipping). Maybe it's because I'm paying by debit card rather than credit card, or maybe the minimum order only applies to non account holders.Something to try for the hot end adhesive is oven glass door sealant. It's fine up to around 300°c, but you do need to add it all in one go because it doesn't like bonding to itself.

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How about only machining the bore for the resistor the length of the resistor + a little bit, cutting a hole through the other side for the wire, cutting a retaining groove and using a circlip to hold the resistor in place?

Threading the thermistor hole, and drill a grub screw to allow the cables out.

Hi, just read through your blog from the beginning over the last few days, really interesting stuff. In an much earlier post you mentioned that you tried to prepare your Mendal in time for a show, are you planning to exhibit anywhere in the future? I've no space to attempt a Reprap myself, but would love to see one working in person.

It was actually BodgeIt who was planning to have a stall at the Makerfaire. It looked like he might not have anything ready in time so I volunteered the Mendel I was building at the time. Unfortunately I didn't get it finished in time, in fact it took about 3 more weeks!

I am trying to test some variants of extruder. The problem i am facing atm is how short the transition zone can be, in the context of ensuring no reverse-flow of the plastic occurs. Any ideea please about what is needed to block the plastic from flowing over/upwards, in the minimal height terms?

Hi, great projectJust thought of a couple ideas you may want to try out or think about.Electric current to heat your extruder. Very high current, very low voltage like is done in a soldering gun. Example: the medal barrel of your extruder could be part of the one turn loop of the secondary of a transformer. You would just need to make the extruder barrel the highest resistance part of the loop so heating is concentrated there.

The same idea a different way. Circular two sided copper clad board. Remove a little copper on one side so the remaining copper looks like pieces of a pie slice. At the tip of each piece is a different size plated through hole for plastic extrusion. Around the edge could be many pairs of holes for large gauge copper wire to complete the secondary one turn loop of each extruder hole in the center.

Yes I have considered induction heating. Because brass and aluminium barrels are such good conductors it needs to be high frequency to exploit the skin effect. I rejected the extra complexity as I don't see any clear advantage over a block of aluminium with a resistor in it.

FR4 PCB material cannot handle the extrusion temperature for more than a few seconds. You probably need to heat more than just the last 1.6mm length of the plastic as well. It needs to be fully molten before it reaches the nozzle aperture.